Radio components are some of the most expensive parts of radio frequency (RF) communications equipment. This is particularly the case in cordless or wireless telephone. In RF communications, and particularly, in cordless or wireless telephone, costs and operational requirements can be very important to the success of communications equipment designs. Engineering such equipment often involves design constraints imposed or dictated by costs of components or by operational requirements. Such costs and operational requirements are particularly important considerations when communications equipment is intended for consumer-level uses and household-type markets.
Various standards, established by the RF communication technology, industry and other sources, often dictate aspects of performance and equipment requirements. Standards have been established, for example, for cordless or wireless telephone products and other communications devices. Certain of the most common standards of the cordless telephone industry include the Cordless Telephone Second Generation (CT2) standard, the European Conference of Postal and Telecommunications Administrations (CEPT) standard which is also referred to as the Cordless Telephone First Generation (CT1) standard, the Cordless Telephone First Generation Plus (CT1+) standard, and the digital European Cordless Telecommunications (DECT) standard, among others.
The CT2 standard, for example, employs a time division duplex (TDD) system and methodology. In TDD, transmit and receive communications occur among two stations, such as a handset and base set unit of a cordless telephone, in a burst manner at distinct intervals of time. In the past, devices conforming to CT2 have transmitted and received over an identical carrier frequency within the bandwidth dictated by the standard. Communications have been possible in TDD units because different time intervals are employed for transmissions and receptions by each station. During an interval that one station is transmitting, the other is receiving, and vice versa, all over the same bandwidth. Devices built according to the CT2 standard have been considered lower-end devices, that is, the devices are typically low-cost to consumers. This low cost is partly attributable to the use in those devices of only a single radio front end. That is possible in CT2 devices because communications occur over the same carrier frequency in the TDD manner. The prior TDD devices, however, at least those devices conforming to the CT2 standard, have implemented the TDD methodology using a single carrier frequency. It has previously been thought that use of limited bandwidth through implementation of TDD methodology over a single carrier frequency provides the greatest advantages. This has not necessarily resulted, however, in the lowest cost for the prior TDD devices.
Other cordless telephone standards, such as the CT1 standard, at times have employed a frequency division duplex (FDD) concept. In typical FDD, transmit and receive communications occur over two distinct, separate carrier channels. Thus, two FDD communications stations, such as, for example, a handset unit and a base set unit of a cordless telephone, each transmit and receive over different carrier channels. While a first unit is transmitting over a particular channel, the second unit is receiving on that same channel. The second unit transmits on a different channel, and the first unit receives on that different channel. FDD systems have tended to be more expensive than TDD systems because additional radio front end components have been required in prior FDD systems in order to accomplish the transmissions and receptions over the separate channels.
In addition, FDD systems use more power to operate the additional circuitry necessary to utilize multiple carrier frequencies as compared to TDD systems. This additional disadvantage for FDD systems contributes to a reduction in the useful battery life for the handset in FDD systems as compared to TDD systems. Providing a cordless communication system with the longest possible useful battery life is a desirable design goal of all cordless communication systems.
FDD systems, however, possess an advantage over TDD systems in that the addition of the additional communication channel in the FDD system allows the communications data rate for each of the channels to be one-half of the rate used by the TDD system that provides the same total communication capacity. The halving of the data rate used by each channel increases the energy per bit provided by the FDD system. This in turn permits the link quality to be more robust than a TDD system operating in the same environment. This reduction in the bit error rate for the FDD channel increases the effective usable range of the cordless communications system by providing an improved signal-to-noise ratio.
There are certain advantages in selecting a particular type of radio frequency communications and, in particular, for communications involving cordless telephone. For example, TDD methods can be advantageous because of the minimal spectrum necessary for such communications and the reduced power consumption for these systems. FDD methods provide advantages of continuous transmission and reception while permitting lower data rates which in turn provide lower bit error rates and correspondingly greater operating range. Traditionally, system requirements lead to selection of one type, along with its advantages, and not the advantages of the other type.